Conventional approaches to battery pack design typically separate the architecture and control of the battery pack from the DC-DC power converter design, architecture and control. The DC-DC converter is used for voltage regulation to the rest of the system or to the load, e.g. to the power inverter that drives the vehicle motor. This results in additional electronic circuits to monitor battery health and faults and balance different battery cells in the battery pack, and in the inability to effectively decouple battery cells performances from each other. A conventional battery pack may have N cells connected in series in a string. Several strings could also be connected in parallel to achieve higher capacity. Each cell or combination of cells has electronic circuit and a Balancing Circuit (B.C.) in order to perform a cell charge balancing function. A balancing controller controls the operation of these balancing circuits in order to try to maintain equal State-Of-Charge (SOC) for the cells. Larger number of balancing circuits for larger number of cells results in higher cost and complexity. A balancing controller utilizes the SOC information to control the balancing circuits for equal SOC. The BC is provided to accommodate non-uniform aging of the cells, non-uniform performance degradation, and non-uniform discharge/discharge in order to achieve battery pack with longer operational life and better performance.
The battery pack output voltage supplies a DC-DC power converter in order to regulate the voltage which will be used as an input to the rest of the system or to the load. A controller measures the DC-DC power converter voltages and currents in order to provide control signals for proper operation. In the conventional battery pack, a degraded cell could impact the whole battery pack performance, resulting in shorting the other cells life and generating additional heat. The balancing circuits and controller could partially take care of this issue if the mismatch between cells is within a limited range. However, a very bad cell (or cells) will result in balancing circuits to keep passing charges between cells which causes additional power losses and heat, eventually leading to overall battery pack degradation (i.e., a worse State-Of-Health, SOH). This affects both the battery pack discharging operation mode and charging operation mode.
Conventionally, there are several methods that can be used to estimate SOC. The cell Open Circuit Voltage (OCV) is a parameter that could be used, either alone or as one of the variables used in a more complicated scheme. Therefore, the determination of the OCV affects the accuracy of the SOC estimation. While it is possible to estimate the OCV by measuring the cell voltage at different cell current values and with the knowledge of the cell impedance, this method does not have good accuracy because it requires the accurate knowledge of the cell impedance and because this impedance value might be affected for different current values, different temperatures, different cells, and as non-uniform aging of cells. This makes the accurate estimation of the OCV during the system operation difficult and inaccurate, and it requires additional electronic circuits and controller.
Conventionally, there are several methods that could be used to determine SOH. An estimate of the cell impedance (Zcell) change is a characteristic of these methods. One fairly complex method to estimate the complete impedance (and not only the DC resistance) involved the application of a periodic time-varying (e.g. sinusoidal) small voltage or current and measuring the corresponding current or voltage, respectively. By using Ohm's Law and the phase shift between the voltage and current, the impedance value can be calculated. However, this method requires added circuits to apply the time-varying signal, which increase complexity and cost.